The present invention relates to semiconductor devices and more particularly to a semiconductor device having a solid-state image sensor.
In CMOS (Complementary Metal Oxide Semiconductor) image sensors, each unit pixel includes a microlens, a red, green or blue color filter, and a photoelectric transducer such as a photodiode. The wavelength of light passing through the color filter of a unit pixel and arriving at the photoelectric transducer depends on the filter color. The light which has entered the photoelectric transducer through the color filter of the unit pixel is converted into an electrical signal and the electrical signal is detected inside the unit pixel. An image sensor in which a plurality of unit pixels with different color filters are arranged over a semiconductor substrate and spaced from one another is disclosed, for example, in Japanese Unexamined Patent Publication No. 2006-279048.
In an image sensor, the light passing through a red color filter has a long wavelength and reaches deep inside a laminated structure, for example, of silicon. Therefore, this light gets into a deeper region (near to the bottom surface) of a semiconductor substrate, for example, of silicon. Some of the electrons generated by photoelectric conversion of the red light which has arrived at the photoelectric transducer are collected by the unit pixel having the red filter by diffusion or drifting. However, it may happen that some of the electrons generated by photoelectric conversion of the red light are wrongly collected by an adjacent unit pixel having, for example, a green color filter by diffusion or drifting. This phenomenon is called electrical crosstalk. Or the light which has passed through the red color filter may accidentally get into an adjacent unit pixel having a green color filter. This phenomenon is called optical crosstalk. Electrical crosstalk and optical crosstalk are collectively called inter-pixel crosstalk. Inter-pixel crosstalk may cause a color tone defect or image quality deterioration in the CMOS image sensor.
In the image sensor described in Japanese Unexamined Patent Publication No. 2006-279048, in particular an upper substrate region located under a pixel with a long wavelength red color filter is electrically insulated from an adjacent pixel with a green color filter and extends more outward than the red pixel in a plan view. The electric charge which has been generated from the light passing through the red color filter and has arrived at the upper substrate region and is collected in the red pixel without entering the green pixel, even if it flows to the outside of the red pixel. Electrical crosstalk is thus prevented.
However, even when the above technique is used, there remains a possibility that inter-pixel crosstalk, in particular optical crosstalk, occurs. With this background, Japanese Unexamined Patent Publication No. 2007-227761 and Japanese Unexamined Patent Publication No. 2008-10544 disclose image sensors (solid-state image sensors) in which neighboring unit pixels are electrically insulated from each other by an isolation insulating film inside which an airspace called an air gap is formed. Also, Japanese Unexamined Patent Publication No. 2009-267208 discloses a flash memory in which an air gap is formed in an isolation insulating film between neighboring cells. Also, Japanese Unexamined Patent Publication No. 2002-203896 discloses a method for manufacturing a shallow trench structure with an air gap.